Is a cheap U-lock good enough?

It depends on your bike's value and where you park. OutdoorGearLab found the budget Kryptonite Kryptolok Standard had the thinnest shackle of Kryptonite's U-locks and was the quickest to cut, around 10 seconds with an angle grinder. r/bikecommuting consensus is that any decent U-lock makes a mid-value bike a less attractive target than the unlocked bike next to it, so a budget U-lock beats a cable lock by a wide margin.